Nitride semiconductors, such as GaN, AlN, and InN, and mixed crystals thereof have a wide band gap and are used for high output electronic devices and/or short-wavelength light emitting devices. Among those mentioned above, as for a high output device, a technique relating to a field-effect transistor (FET) and, in particular, a technique relating to a high electron mobility transistor (HEMT) have been developed (for example, see Japanese Laid-open Patent Publication No. 2011-3652). The HEMT using a nitride semiconductor as described above is used for a high output/high efficiency amplifier, a high power switching device, or the like.
Incidentally, in the HEMT using a nitride semiconductor, holes are generated and accumulated in an electron transit layer functioning as a channel by impact ionization in a high electric field. When holes generated by impact ionization in a high electric field are accumulated in the electron transit layer, for example, a decrease in withstand voltage of a semiconductor device, fluctuation in drain conductance properties caused by a kink effect, and a decrease in switching speed may arise. Hence, as a method for suppressing those disadvantages, extraction of holes generated by impact ionization from the electron transit layer is effective, and Japanese Laid-open Patent Publication No. 2011-3652 has disclosed a semiconductor device having the structure in which holes generated in the electron transit layer by impact ionization are extracted.
In particular, in the semiconductor device disclosed in Japanese Laid-open Patent Publication No. 2011-3652, as depicted in FIG. 1, a substrate 910 formed of GaN or the like and having a surface along the (000-1) plane is used. In addition, on part of the substrate 910, a Ga-surface forming layer 911 formed of AlN or the like and having a surface along the (0001) plane is provided. Accordingly, on the surface of the substrate 910 on which the Ga-surface forming layer 911 is partially formed, an electron transit layer 921 is formed from GaN by crystal growth. In this case, in a region of the substrate 910 in which the Ga-surface forming layer 911 is formed, the electron transit layer 921 formed of the crystal grown GaN has a surface along the (0001) plane and includes many Ga atoms at the surface. In this application, the surface including many Ga atoms as described above will be referred to as a Ga-polar surface in some cases. On the other hand, in a region in which the Ga-surface forming layer 911 is not formed, the electron transit layer 921 formed of the crystal grown GaN has a surface along the (000-1) plane and includes many N atoms at the surface. In this application, the surface including many N atoms as described above will be referred to as an N-polar surface in some cases.
Next, on the electron transit layer 921 thus formed, an electron supply layer 922 of AlGaN is formed. Furthermore, on the electron supply layer 922 in a region in which the Ga-surface forming layer 911 is formed, a gate electrode 941, a source electrode 942, and a drain electrode 943 are formed, and on the electron supply layer 922 in a region in which the Ga-surface forming layer 911 is not formed, a hole extraction electrode 944 is formed. Since many Ga atoms are present at the surface of the electron transit layer 921 in the region in which the Ga-surface forming layer 911 is formed, in the electron transit layer 921 in the vicinity of the interface between the electron transit layer 921 and the electron supply layer 922, a two-dimensional electron gas (hereinafter referred to as “2DEG”) 921a is generated. In addition, since many N atoms are present at the surface of the electron transit layer 921 in the region in which the Ga-surface forming layer 911 is not formed, in the electron transit layer 921 in the vicinity of the interface between the electron transit layer 921 and the electron supply layer 922, a two-dimensional hole gas (hereinafter referred to as “2DHG”) 921b is generated.
Since the 2DHG 921b is generated in the electron transit layer 921 as described above, in the electron transit layer 921, holes generated by impact ionization can be extracted through the hole extraction electrode 944. Accordingly, the decrease in withstand voltage in a semiconductor device, the fluctuation in drain conductance properties caused by a kink effect, the decrease in switching speed, and the like can be suppressed.
In addition, besides the above Japanese Laid-open Patent Publication No. 2011-3652, the following non-patent literatures have also been discussed in this application. As the Non-Patent Literatures, for example, there may be mentioned O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, W. J. Schaff, and L. F. Eastman, R. Dimitrov, L. Wittmer, and M. Stutzmann, W. Rieger and J. Hilsenbeck, J. Appl. Phys. Vol. 85 (1999) 3222; N. F. Gardner, J. C. Kim, J. J. Wierer, Y. C. Shen, and M. R. Krames, Appl. Phys. Lett. 86 (2005) 111101; P. Waltereit, O. Brandt, M. Ramsteiner, R. Uecker, P. Reiche, K. H. Ploog, J. Cryst. Growth 218 (2000) 143; Atsushi Kobayashi, Satoshi Kawano, Yuji Kawaguchi, Jitsuo Ohta, and Hiroshi Fujioka, Appl. Phys. Lett. 90 (2007) 041908; R. Armitage and H. Hirayama, Appl. Phys. Lett. 92 (2008) 092121; Koji Okuno, Yoshiki Saito, Shinya Boyama, Naoyuki Nakada, Shugo Nitta, Ryoichi George Tohmon, Yasuhisa Ushida, and Naoki Shibata, Appl. Phys. Express 2 (2009) 031002; X. Ni, M. Wu, J. Lee, X. Li, A. A. Baski et al. Appl. Phys. Lett. 95 (2009) 111102; Masayuki Kuroda, Hidetoshi Ishida, Tetsuzo Ueda, and Tsuyoshi Tanaka, J. Appl. Phys. 102 (2007) 093703; R. Schuber, M. M. C. Chou, P. Vincze, Th. Schimmel, and D. M. Schaadt, AIP Conf. Proc. 1399 (2011) 191; and M. D. Craven, F. Wu, A. Chakraborty, B. Imer, U. K. Mishra et al. Appl. Phys. Lett. 84 (2004) 1281.
However, according to a semiconductor device having the structure as described above, in the electron transit layer 921 and the like, dislocations, lattice defects, and the like are liable to occur in a boundary 931 portion between the region in which the Ga-surface forming layer 911 is formed and the region in which the Ga-surface forming layer 911 is not formed.
When dislocations and lattice defects are generated in the electron transit layer 921 and the like as described above, since holes are trapped in the dislocations and/or the lattice defects, it becomes difficult to extract holes, and as a result, for example, a high voltage is inevitably applied to the hole extraction electrode 944.